It is common practice to form a seal between the wall of an opening and a shaft or other cylindrical member extending through the opening by positioning a packing member between the wall of the opening and the outer surface of the member. Packing members are typically annular bodies having a generally V-shaped or truncated V-shaped cross-section. Packing members are, in many cases, formed from an elastomeric material such as rubber or plastic. One such packing member is disclosed in U.S. Pat. No. 4,526,385, issued Jul. 2, 1985.
The operation of a device such as a hydraulic cylinder, typically involves both static and dynamic forces generated by the friction between the packing member and the shaft. Static friction must be overcome during the initial actuation of the cylinder whereas dynamic friction is encountered during continued movement of the shaft. The amount of friction between the shaft and the packing member is determined by a number of factors including the amount of surface area of the packing member in contact with the shaft. Preferably, the amount of friction between the shaft and the packing member is minimized while maintaining an effective seal.
The amount of force required to overcome static friction and initially move the shaft is typically greater than the force required to overcome dynamic friction after the shaft has started moving. Since the amount of force required to overcome static friction is greater than the amount of force needed to overcome dynamic friction, static friction is an important consideration in the selection and design of packing members. Moreover, during the operation of a hydraulic cylinder, a large difference between the force required to overcome static friction and the force required to overcome dynamic friction may result in the cylinder jerking or "stuttering" during operation. Large hydraulic and mechanical stresses can be generated when a hydraulic cylinder jerks in such a manner, stresses that may damage or cause excessive wear of system components, for example hydraulic valves, fittings and the hydraulic pump supplying pressurized fluid to the cylinder. Consequently, it is desirable not only that the friction between packing member and a slidable member passing through the packing member be minimized, but also that the difference in the amount of force required to overcome static versus dynamic friction be minimized.